1. Field of the Invention
The present invention relates generally to high voltage circuits. More specifically, the present invention relates to high voltage circuits in an electronic erasable programmable read only memory (EEPROM) integrated circuit (IC).
2. Related Art
Metal-oxide semiconductor field effect transistors (MOSFETs) are manufactured such that they can withstand a particular maximum voltage across any two terminals of the MOSFET. A MOSFET having a maximum voltage of V.sub.max is "rated" at a voltage of V.sub.max. The maximum voltage of a MOSFET is termed the breakdown voltage of the MOSFET. A common MOSFET breakdown voltage is equal to approximately twelve volts. As the rated voltage of a MOSFET increases the associated expense in manufacturing the IC containing the MOSFET increases. The increased manufacturing expenses may be particularly acute with p-channel MOSFETs. For example, in order to manufacture an IC containing a p-channel MOSFETs rated at twenty-four volts, a significant increase in process technology and materials is required when compared to an IC containing only twelve volt rated p-channel MOSFETs.
When operating an IC it is often necessary to apply a voltage greater than twelve volts to one or more MOSFETs in the IC. Typically, higher rated MOSFETs are used in ICs to which voltages greater than twelve volts are applied. However, as discussed above, the use of such higher rated MOSFETs are not always desirable since they are more difficult and more expensive to produce.
One type of circuit which, for proper operation, requires a maximum voltage greater than twelve volts is an electronic erasable programmable read only memory (EEPROM) circuit. A general description of an EEPROM circuit 100 and how it operates is given below with reference to FIG. 1.
The EEPROM circuit 100 includes an array of EEPROM cells 102, hereafter "memory cells". These memory cells 102 are non-volatile. In other words, each memory cell 102 can store a bit of dam for ten years or more. In addition, these memory cells 102 can be erased and programmed by a user. Typically, each memory cell includes a transistor. Nodes of the transistor are connected to a row line 104, a column line 108, an erase line 106, and a ground line 110. The operation of memory cell 102 will now be described with reference to FIG. 2, which shows one of the memory cells 102 in greater detail.
In order to properly function, a memory cell 102 is typically erased, programmed and then read based on instructions output by a controlling processor (not shown). When erasing a memory cell 102, a voltage of approximately twenty-two volts is applied to the erase gate 206 via the erase line 106. When erasing a memory cell 102, the row line 104 and the column line 108 are typically "low", i.e., approximately zero volts or ground. The high voltage applied to the erase gate 206 causes a positive charge to form on the floating gate 204 of the memory cell 102. This positive charge on the floating gate 204 effectively erases the memory cell 102. That is, the memory cell is considered to store a binary "zero" when the floating gate 204 stores a positive charge. Typically, all memory cells 102 are erased before the processor (not shown) requests that some memory cells 102 be programmed to store a binary "one".
Programming a memory cell 102 typically requires that a twelve volt signal be applied to the control gate 202 of the memory cell 102 via the row line 104 while a signal having a voltage between seven and eight volts is applied to the drain 210 of the transistor within the memory cell 102 via the column line 108. The source 208 of the transistor in the memory cell 102 is held to zero volts or ground. When these potentials are applied to the memory cell the positive charge stored in the floating gate 204 during the memory cell erase operation, described above, is reduced. When the charge stored by the floating gate 204 is below a predetermined level, the memory cell 102 is considered to store a binary "one". After erasing and programming the memory cells 102, the memory cells 102 can be read. Reading a memory cell 102 is generally accomplished by applying a voltage of approximately five volts to the control gate 202 of the memory cell 102 via the row line 104 and applying a voltage of approximately one and one-half volts to the drain. Further details regarding the operation and structure of an EEPROM will be apparent to persons skilled in the relevant art.
FIG. 3 illustrates an EEPROM IC 300. Typically, EEPROM ICs do not contain the VOLTAGE REGULATOR INPUT SWITCH 318. A processor (not shown) can request that the EEPROM IC 300 erase, program or read the memory cells 102 of an EEPROM circuit 100, as described above. The processor provides a V.sub.pp SIGNAL 302, a V.sub.dd signal 304, ERASE CONTROL SIGNALs 308, a READ-OR-PROGRAM SIGNAL 314, and a PROGRAM signal 326 to the EEPROM IC 300. The value of these signals varies depending upon which operation the processor requests.
To request the erasing of an EEPROM circuit 100, the processor (not shown) outputs a potential of approximately twenty-four volts on the V.sub.pp SIGNAL 302 and a potential of approximately five volts on the ERASE CONTROL SIGNAL 308 associated with the EEPROM circuit 100 to be erased. The V.sub.pp SIGNAL 302 and the ERASE CONTROL SIGNAL 308 are input into a HIGH VOLTAGE SWITCH 306, described below. The HIGH VOLTAGE SWITCH 306 outputs a voltage of approximately twenty-two volts to the ERASE LINE of the EEPROM circuit 100 on output line 310.
To request that memory cell 102 within an EEPROM circuit 100 be programmed, the processor (not shown) outputs a potential of approximately twelve volts on the V.sub.pp SIGNAL 302, a potential of approximately five volts on the V.sub.dd SIGNAL 304 and a potential of approximately five volts on the V.sub.pp SWITCH INPUT LINE 314. The V.sub.pp SWITCH 312 outputs a potential of approximately V.sub.pp volts on its output line 316 when the V.sub.pp SWITCH INPUT LINE 314 is high, i.e., the signal voltage is approximately five volts. As stated above, when programming the EEPROM circuit, V.sub.pp is approximately twelve volts. Typical EEPROM ICs do not include the VOLTAGE REGULATOR INPUT CIRCUIT 318 which in the present invention ensures that the VOLTAGE REGULATOR 324 input signal VPPZ will not have a voltage which exceeds twelve volts. Instead, the VPPZ SIGNAL 322 is input directly into VOLTAGE REGULATOR 324 along with the PROGRAM signal 326 and the V.sub.dd SIGNAL 304. When programming a memory cell 102, the VOLTAGE REGULATOR outputs a signal having a voltage between seven volts and eight volts on the VOLTAGE REGULATOR OUTPUT LINE 328. This output signal is sent to COLUMN PROGRAM CIRCUITS (not shown) within the EEPROM circuit 100.
As discussed above, to request that the EEPROM circuit 100 be read, the processor outputs a potential of approximately five volts on the V.sub.pp SIGNAL 302 and a potential of approximately five volts on the V.sub.dd SIGNAL 304. The detailed operation of the EEPROM IC 300 during a READ operation will be apparent to persons skilled in the relevant art.
In the typical EEPROM IC 300, the HIGH VOLTAGE SWITCH 306, the V.sub.pp SWITCH 312, and the VOLTAGE REGULATOR 324 will all be exposed to a potential of approximately twenty-four volts when the processor (not shown) requests that an EEPROM circuit 100 be erased, as described above. When a twenty-four volt potential is applied to a circuit, the high voltage circuits 306, 312, and 324 are typically designed such that some p-channel transistors within the high voltage circuits 306, 312, and 324 have a rating of, at least, twenty-four volts. A typical HIGH VOLTAGE SWITCH 306, V.sub.pp SWITCH 312, and VOLTAGE REGULATOR 324 will now be described in greater detail.
In FIGS. 4-6, a transistor having a circle at its control gate is a p-channel metal oxide semiconductor field effect transistor (MOSFET). If the transistor does not have a circle at its control gate then it is an n-channel MOSFET. If the MOSFET has an "X" it its channel then it is a twenty-four volt rated MOSFET, otherwise it is a twelve volt rated MOSFET. If the back-gate connection is not explicitly shown, it is connected to V.sub.ss, or ground, if the transistor is an n-channel MOSFET or to V.sub.pp if the transistor is a p-channel MOSFET.
FIG. 4 illustrates a schematic of a typical HIGH VOLTAGE SWITCH 306. Transistors 402, 406, and 408 are each n-channel MOSFETs which are rated at twenty-four volts. Transistor 404 is a p-channel MOSFET rated at twenty-four volts. Additionally, transistor 404 is a "weak" transistor, i.e., when the transistor conducts it has a high resistance.
There can be many EEPROM circuits 100 in an EEPROM IC 300. A HIGH VOLTAGE SWITCH 306 is typically associated with each EEPROM circuit 100, as shown in FIG. 3. To request that an EEPROM circuit be erased, the processor (not shown) outputs a twenty-four volt potential on the V.sub.pp SIGNAL 302, as described above. When the input signal voltage, IN, is "high", i.e., approximately five volts, the output signal 310 voltage of the HIGH VOLTAGE SWITCH 306 is approximately equal to V.sub.ss. When the input signal voltage, IN, is "low", i.e. approximately zero volts, the output signal voltage of the HIGH VOLTAGE SWITCH 310 is approximately equal to: V.sub.pp minus the threshold voltage of transistor 406. Transistor threshold voltages are described below. Further details pertaining to the structure and operation of the circuit shown in FIG. 4 will be apparent to persons skilled in the relevant art.
The reason why the p-channel MOSFET 404 which is rated at twenty-four volts 404 is used in this circuit shall now be described. If the input, IN, is "high", MOSFET 402 conducts, thereby pulling the voltage on node N10 down to approximately zero volts. Simultaneously, p-channel MOSFET 404 also conducts, since its gate input voltage, i.e., five volts, is significantly less than its drain voltage, i.e., twenty-four volts. However, because transistor 404 is a weak device, as described above, transistors 404 and 402 will act as a voltage divider having a significant voltage drop across transistor 404. Therefore, the voltage at node N10 will be close to zero volts. In this situation, the voltage drop across the p-channel transistor 404 is approximately twenty-four volts. Therefore, transistor 404 must be rated at a minimum of twenty-four volts in order to prevent transistor 404 from breaking-down.
The HIGH VOLTAGE SWITCH circuit 306 could alternatively be designed using a well known charge-pump circuit (not shown). Such a circuit does not require a p-channel MOSFET rated at twenty-four volts. However, the circuit does require n-channel MOSFETs rated at twenty-four volts whose characteristics, e.g., threshold voltage, are known with high precision. Such n-channel transistors are expensive to manufacture and, therefore, circuits requiring such precise transistors are not desirable.
As stated above, it is often unacceptable to design a circuit within an EEPROM IC 300 which requires the use of p-channel MOSFETs rated higher than twelve volts. Therefore, a circuit performing the same functions as the HIGH VOLTAGE SWITCH 306, described above, which does not employ any p-channel MOSFETs rated higher than twelve volts is desirable.
FIG. 5 is a schematic of a typical V.sub.pp SWITCH 312. Transistor 502 is a twenty-four volt rated p-channel MOSFET. Transistors 504 and 506 are twenty-four volt rated n-channel MOSFETs. Transistor 508 is a twelve-volt rated p-channel MOSFET. Additionally, transistor 508 is a weak transistor.
The V.sub.pp SWITCH 312 outputs a voltage signal VPPW which is equal to the voltage of the V.sub.pp, SIGNAL 302 when the V.sub.pp SWITCH input, IN 314, is high. The input, IN 314, is high only when the processor has requested that the EEPROM circuit 100 be programmed or read. That is, when the processor requests that an erase be performed on the EEPROM circuit 100, and consequently the voltage on the V.sub.pp SIGNAL 302 is approximately twenty-four volts, the input, IN 314, is low. When the input, IN, is low, the output signal VPPW is approximately equal to the voltage on the V.sub.dd SIGNAL 304, i.e., five volts. Further details pertaining to the structure and operation of the circuit shown in FIG. 5 will be apparent to persons skilled in the relevant art.
As stated above, when the V.sub.pp SWITCH input, IN 314, is low, and V.sub.pp SIGNAL 302 has a voltage of approximately twenty-four volts, the voltage of the output signal VPPW is equal to V.sub.dd, i.e., approximately five volts. Therefore, the voltage drop across the source and the drain of transistor 502, and across the control gate and the drain of transistor 502 is approximately nineteen volts, i.e., (V.sub.pp -V.sub.dd). Therefore, transistor 502 must be rated above twelve volts in order to prevent the transistor 502 from breaking down.
The V.sub.pp SWITCH 312 could alternatively be designed using a well known charge-pump circuit (not shown). Such a circuit does not require a p-channel MOSFET rated at twenty-four volts. However, the circuit does require n-channel MOSFETs rated at twenty-four volts whose characteristics, e.g., voltage threshold, are known with high precision. Such n-channel transistors are expensive to manufacture and are, therefore, undesirable.
As stated above, it is often unacceptable to design a circuit within an EEPROM IC 300 which requires the use of a twenty-four volt rated p-channel MOSFET. Therefore, a circuit performing the same functions as the V.sub.pp SWITCH 312, described above, which does not employ any p-channel MOSFETs rated higher than twelve volts is desirable.
A typical VOLTAGE REGULATOR 324 is shown in FIG. 6A and FIG. 6B. FIG. 6A is the portion of the VOLTAGE REGULATOR 324 which performs the functions of the VOLTAGE REGULATOR INPUT CIRCUIT 318 illustrated in FIG. 3. The output VPPZ is input into the portion of the VOLTAGE REGULATOR 324 shown in FIG. 6B. Transistors 602, 604, 606 and 608 are twenty-four volt rated p-channel MOSFETs. Transistors 610, 612 and 614 are twenty-four volt rated n-channel MOSFETs. Transistor 618 is a twelve volt rated p-channel MOSFET. Device 616 is an inverter.
FIG. 6A illustrates a logic circuit which has as its input, IN, a NOT-PROGRAM signal, i.e., the inverted PROGRAM signal 326. When the input, IN, is high, a voltage of twenty-four volts can be input into the circuit via the V.sub.pp SIGNAL 302. In this situation the circuit shown in FIG. 6A will output a twelve volt signal at node VPPZ. When the input, IN, is low, a voltage of twelve volts, V.sub.pp, must be input into the circuit via V.sub.pp SIGNAL 302. In this situation the circuit shown in FIG. 6A will output a twelve volt signal at node VPPZ. Further details pertaining to the structure and operation of the circuit shown in FIG. 6A will be apparent to persons skilled in the relevant art.
The twenty-four volt rated p-channel MOSFETs 602, 604, 606, and 608 are used in FIG. 6A because the voltage across their source and drain can be above twelve volts. For example, when the input, IN, is high, i.e., approximately five volts, and the V.sub.pp SIGNAL voltage is twenty-four volts, transistor 602 conducts. The difference between the transistor's 602 drain voltage and the transistor's 602 control gate voltage is approximately nineteen volts, i.e., (V.sub.pp -V.sub.dd). Therefore, transistor 602 must be rated above twelve volts in order to prevent the transistor from breaking-down.
As stated above, it is often unacceptable to design a circuit within an EEPROM IC 300 which requires the use of p-channel MOSFETs rated above twelve volts. Therefore, a circuit performing the same functions as the VOLTAGE REGULATOR 324 described above, which does not employ any p-channel MOSFETs rated higher than twelve volts is desirable.
The VOLTAGE REGULATOR 324 shown in FIG. 6B provides an output signal V7 having a voltage between seven volts and eight volts when the processor (not shown) requests that a programming operation be performed on an EEPROM circuit 100. Transistors 632 and 634 are five volt rated n-channel MOSFETs. The remaining transistors are twelve volt rated n-channel MOSFETs. Blocks 620 and 636 are well known voltage dividers. No twenty-four volt rated MOSFETs are necessary since the input, VPPZ, never exceeds twelve volts because VPPZ is output from the circuit illustrated in FIG. 6A which limits the voltage on the VPPZ signal.
The VOLTAGE REGULATOR 328 inputs the PROGRAM signal 326, shown as INX in FIG. 6B. The input, INX, is high when the processor (not shown) requests that a memory cell be programmed, as discussed above. The VOLTAGE REGULATOR output signal V7 has a voltage between seven volts and eight volts when the input, INX, is high. The transistors 622-630 of the voltage divider 620 are chosen such that the size, and therefore, the resistance between the drain and the source, of transistor 622 when compared to the size of transistors 624-630 enables the output signal V7 voltage to be within the acceptable output range, i.e., between seven volts and eight volts. Threshold voltages can vary between transistors on different integrated circuit chips, even when the transistors have the same nominal threshold voltage.
Transistor 622 is manufactured having a nominal threshold voltage, e.g., 2.5 volts. In an n-channel MOSFET, the transistor 622 conducts if a control gate voltage is at least one threshold voltage V.sub.TH above the source voltage of the transistor, i.e., the voltage on signal V7. As discussed above, the threshold voltage can vary from the nominal threshold voltage. The output signal V7 voltage is affected by the threshold voltage variations in transistors 622 and 640. A VOLTAGE REGULATOR 322 which is more independent of threshold voltage variations of individual transistors is desirable. Further details pertaining to the structure and operation of the VOLTAGE REGULATOR of FIG. 6A and 6B will be apparent to persons skilled in the relevant art.
What are required are a HIGH VOLTAGE SWITCH, a V.sub.pp SWITCH, and a VOLTAGE REGULATOR INPUT CIRCUIT for use in an EEPROM IC 300 which do not require the use of a p-channel MOSFETs rated above twelve volts. In addition, what is needed is a VOLTAGE REGULATOR which more precisely regulates its output voltage when the MOSFETs which comprise the VOLTAGE REGULATOR have threshold voltages which deviate from their nominal threshold voltages.